The design and fabrication of micrometer sized structures and devices have had an enormous impact on a number of important technologies including microelectronics, optoelectronics, microfluidics and microsensing. The ability to make micro-sized electronic devices, for example, has revolutionized the electronics field resulting in faster and higher performing electronic components requiring substantially less power. As these technologies continue to rapidly develop, it has become increasingly apparent that additional gains are to be realized by developing the ability to manipulate and organize matter on the scale of nanometers. Advances in nanoscience and technology promise to dramatically impact many areas of technology ranging from materials science to applied engineering to biotechnology.
Fabrication of devices having nanoscale dimensions is not merely a natural extension of the concept of miniaturization, but a fundamentally different regime in which physical and chemical behavior deviates from larger scale systems. For example, the behavior of nanoscale assemblies of many materials is greatly influenced by their large interfacial volume fractions and quantum mechanical effects due to electronic confinement. The ability to make structures having well-defined features on the scale of nanometers has opened up the possibility of making devices based on properties and processes only occurring at nanometer dimensions, such single-electron tunneling, Coulomb blockage and quantum size effect. The development of commercially practical methods of fabricating sub-micrometer sized structures from a wide range of materials, however, is critical to continued advances in nanoscience and technology.
Photolithography is currently the most prevalent method of microfabrication, and nearly all integrated electronic circuits are made using this technique. In conventional projection mode photolithography, an optical image corresponding to a selected two dimensional pattern is generated using a photomask. The image is optically reduced and projected onto a thin film of photoresist spin coated onto a substrate. Alternatively, in Direct Write to Wafer photolithographic techniques, a photoresist is directly exposed to laser light, an electron beam or ion beam without the use of a photomask. Interaction between light, electrons and/or ions and molecules comprising the photoresist chemically alters selected regions of the photoresist in a manner enabling fabrication of structures having well defined physical dimensions. Photolithography is exceptionally well suited for generating two-dimensional distributions of features on flat surfaces. In addition, photolithography is capable of generating more complex three dimensional distributions of features on flat surfaces using additive fabrication methods involving formation of multilayer stacks.
Recent advances in photolithography have extended its applicability to the manufacture of structures having dimensions in the submicron range. For example, nanolithographic techniques, such as deep UV projection mode lithography, soft X-ray lithography, electron beam lithography and scanning probe methods, have been successfully employed to fabricate structures with features on the order of 10s to 100s of nanometers. Although nanolithography provides viable methods of fabricating structures and devices having nanometer dimensions, these methods have certain limitations that hinder their practical integration into commercial methods providing low cost, high volume processing of nanomaterials. First, nanolithographic methods require elaborate and expensive steppers or writing tools to direct light, electrons and/or ions onto photoresist surfaces. Second, these methods are limited to patterning a very narrow range of specialized materials, and are poorly suited for introducing specific chemical functionalities into nanostructures. Third, conventional nanolithography is limited to fabrication of nanosized features on small areas of ultra-flat, rigid surfaces of inorganic substrates and, thus is less compatible with patterning on glass, carbon and plastic surfaces. Finally, fabrication of nanostructures comprising features having selectable lengths in three dimensions is difficult due to the limited depth of focus provided by nanolithographic methods, and typically requires labor intensive repetitive processing of multilayers.
The practical limitations of photolithographic methods as applied to nanofabrication have stimulated substantial interest in developing alternative, non-photolithographic methods for fabricating nanoscale structures. In recent years, new techniques based on molding, contact printing and embossing collectively referred to as soft lithography have been developed. These techniques use a conformable patterning device, such as a stamp, a mold or mask, having a transfer surface comprising a well defined relief pattern. Microsized and nanosized structures are formed by material processing involving conformal contact on a molecular scale between the substrate and the transfer surface of the patterning device. Patterning devices used in soft lithography typically comprise elastomeric materials, such as poly(dimethylsiloxane) (PDMS), and are commonly prepared by casting prepolymers against masters generated using conventional photolithography. The mechanical characteristics of the patterning devices are critical to the fabrication of mechanically robust patterns of transferred materials having good fidelity and placement accuracy.
Soft lithographic methods capable of generating microsized and/or nanosized structures include nanotransfer printing, microtransfer molding, replica molding, micromolding in capillaries, near field phase shift lithography, and solvent assisted micromolding. Conventional soft lithographic contact printing methods, for example, have been used to generate patterns of self assembled monolayers of gold having features with lateral dimensions as small as about 250 nm. Structures generated by soft lithography have been integrated into a range of devices including diodes, photoluminescent porous silican pixels, organic light emitting diodes and thin-film transistors. Other applications of this technology generally include the fabrication flexible electronic components, microelectromechanical systems, microanalytical systems and nanophotonic systems.
Soft lithographic methods for making nanostructures provide a number of benefits important to fabricating nanoscale structures and devices. First, these methods are compatible with a wide range of substrates, such as flexible plastics, carbonaceous materials, ceramics, silicon and glasses, and tolerate a wide range of transfer materials, including metals, complex organic compounds, colloidal materials, suspensions, biological molecules, cells and salt solutions. Second, soft lithography is capable of generating features of transferred materials both on flat and contoured surfaces, and is capable of rapidly and effectively patterning large areas of substrate. Third, soft lithographic techniques are well-suited for nanofabrication of three dimensional structures characterized by features having selectably adjustable lengths in three-dimensions. Finally, soft lithography provides low cost methods which are potentially adaptable to existing commercial printing and molding techniques.
Although conventional PDMS patterning devices are capable of establishing reproducible conformal contact with a variety of substrate materials and surface contours, use of these devices for making features in the sub −100 nm range are subject to problems associated with pressure induced deformations due to the low modulus (3 MPa) and high compressibility (2.0 N/mm2) of conventional single layer PDMS stamps and molds. First, at aspect ratios less than about 0.3, conventional PDMS patterning devices having wide and shallow relief features tend to collapse upon contact with the surface of the substrate. Second, adjacent features of conventional single layer PDMS patterning devices having closely spaced (<about 200 nm), narrow (<about 200 nm) structures tend to collapse together upon contact with a substrate surface. Finally, conventional PDMS stamps are susceptible to rounding of sharp corners in transferred patterns when a stamp is released from a substrate due to surface tension. The combined effect of these problems is to introduce unwanted distortions into the patterns of materials transferred to a substrate. To minimizing pattern distortions caused by conventional single layer PDMS patterning devices, composite patterning devices comprising multilayer stamps and molds have been examined as a means of generating structures having dimensions less than 100 nm.
Michel and coauthors report microcontact printing methods using a composite stamp composed of a thin bendable layer of metal, glass or polymer attached to an elastomeric layer having a transfer surface with a relief pattern. [Michel et al. Printing Meets Lithography: Soft Approaches to High Resolution Patterning, IBM J. Res. & Dev., Vol. 45, No. 5, pgs 697–719 (September 2001). These authors also describe a composite stamp design consisting of a rigid supporting layer and a polymer backing layer comprising a first soft polymer layer attached to a second harder layer having a transfer surface with a relief pattern. The authors report that the disclosed composite stamp designs are useful for “large area, high-resolution printing applications with feature sizes as small as 80 nm.”
Odom and coauthors disclose a composite, two layer stamp design consisting of a thick (≈3 mm) backing layer of 184 PDMS attached to a thin (30–40 microns), layer of h-PDMS having a transfer surface with relief patterns. [Odem et al., Langmuir, Vol. 18, pgs 5314–5320 (2002). The composite stamp was used in this study to mold features having dimensions on the order of 100 nm using soft lithography phase shifting photolithography methods. The authors report that the disclosed composite stamp exhibits increase mechanical stability resulting in a reduction in sidewall buckling and sagging with respect to conventional low modulus, single layer PDMS stamps.
Although use of conventional composite stamps and molds have improved to some degree the capabilities of soft lithography methods for generating features having dimensions in the sub-100 nm range, these techniques remain susceptible to a number of problems which hinder there effective commercial application for high throughput fabrication of micro-scale and nanoscale devices. First, some conventional composite stamp and mold designs have limited flexibility and, thus, do not make good conformal contact with contoured or rough surfaces. Second, relief patterns of conventional, multimaterial PDMS stamps are susceptible to undesirable shrinkage during thermal or ultraviolet curing, which distort the relief patterns on their transfer surfaces. Third, use of conventional composite stamps comprising multilayers having different thermal expansion coefficients can result in distortions in relief patterns and curvature of their transfer surfaces induced by changes in temperature. Fourth, use of stiff and/or brittle backing layers, such as glass and some metal layers, prevents easy incorporation of conventional composite stamps into preexisting commercial printer configurations, such as rolled and flexographic printer configurations. Finally, use of composite stamps having transfer surfaces comprising high modulus elastomeric materials impede formation of conformal contact between a transfer surface and a substrate surface necessary for high fidelity patterning.
It will be appreciated from the foregoing that there is currently a need in the art for methods and devices for fabricating high resolution patterns of structures having features on the scale of 10s to 100s of nanometers. Specifically, soft lithography methods and patterning devices are needed which are capable of fabricating patterns of nanoscale structures having high fidelity, good mechanical robustness and good placement accuracy. In addition, patterning devices are needed that minimize pattern distortions, for example by reducing relief pattern shrinkage during thermal or ultraviolet curing and/or minimizing temperature induced distortions as compared to conventional patterning devices. Finally, soft lithography methods and devices are needed that are compatible with and can be easily integrated into preexisting high speed commercial printing and molding systems.